Thin film semiconductor device

ABSTRACT

A thin film semiconductor device comprising a substrate having an insulating surface, gate electrodes disposed on the insulating surface, gate insulating films disposed on upper portions of the gate electrodes, and thin film semiconductors disposed on the gate insulating films and including channel forming regions, source regions and drain regions. Two kinds of thin film semiconductor unit are disposed on the substrate. A first thin film semiconductor unit includes the thin film semiconductor of polycrystal, an insulating film covering an upper portion of the channel forming region, impurity semiconductor films doped with trivalent or pentavalent impurities and covering the source region and the drain region, and conductive films disposed on the impurity semiconductor films. A second thin film semiconductor unit includes the thin film semiconductor of amorphous, and other components similar to the first thin film semiconductor unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film semiconductor device inwhich a plurality of thin film semiconductors are formed on a substratehaving an insulating surface, which can be applied to a liquid crystaldisplay device and the like.

2. Description of Related Art

An active matrix liquid crystal display device is known as one ofdevices employing a number of thin film transistors using thin filmsemiconductors. The active matrix liquid crystal display device is alsoreferred to as what is called an AMLCD (Active Matrix Liquid CrystalDisplay), and is classified into some types according to the materialsof the thin film transistors or the structure of the transistors. Sincethe thin film transistor is also referred to as what is called a TFT(Thin Film Transistor), the AMLCD is also referred to as a TFT liquidcrystal.

With respect to the materials, there are an amorphous TFT type usingamorphous silicon as a main material, a high temperature polysilicon TFTtype using polycrystal silicon formed through a high temperature processmore than 1,000° C. as a main material, a low temperature polysiliconTFT type using polycrystal silicon formed through a low temperatureprocess of 600° C. as a main material, and the like. With respect to thestructure of transistors, there are a bottom-gate type in which a gateelectrode is disposed at a lower side, a top-gate type in which a gateelectrode is disposed at an upper side, and the like.

Although there are features in the respective materials or the structureof the transistors, in the case of the amorphous TFT, since the mobilityis small and not larger than 1 cm²/Vsec, in order to form a channelforming region between a gate insulating film and an active siliconlayer with proper coordination, the gate insulating film and the activesilicon layer must be continuously formed, and in order to preventhydrogen in the amorphous silicon from drawing off, the amorphoussilicon can not be heated up to a high temperature over 300° C. afterformation thereof. Further, since the amorphous silicon hasphotosensitivity, it is desired to be as thin as possible, not largerthan 300 Å, so that the bottom gate type reverse stagger structure isused.

Since the amorphous TFT has large resistance at an OFF-state, anOFF-state leak current is small, so that it is most suitable for aswitching transistor of a pixel. However, since the mobility thereof issmall, it is impossible to form a driver circuit such as a shiftregister on a substrate, and an external IC using crystalline silicon isalways required. Accordingly, the amorphous TFT has problems inminiaturization and lowering the cost.

With respect to the high temperature TFT, since a high temperatureprocess over 1,000° C. can be carried out, steps similar to socalledcrystalline silicon can be used. Thus, a very stable process can beconducted and also the mobility thereof is about 100 cm²/Vsec, a drivercircuit can be formed on a substrate. However, as a substrate capable ofbeing used at a high temperature of 1,000° C., a substrate other than anexpensive substrate such as quartz can not be used, so that theenlargement of the substrate is difficult, and the application of thehigh temperature TFT is restricted to a view finder with a diagonal ofat most two inches or the like.

The low temperature polysilicon can be formed with the merits of boththe amorphous TFT and the high temperature polysilicon TFT, and it hassuperior properties, that is, a TFT having a high mobility can be formedon a low cost normal glass substrate. Accordingly, it is also possibleto form a driver circuit on the substrate and at the same time, to forma switching transistor for a liquid crystal pixel.

However, when the low temperature polysilicon is formed, in a step ofcrystallizing the amorphous silicon formed on the glass substrate, it isknown by experiment that when the amorphous silicon film is thin, it cannot be crystallized by the formation through low temperature thermalanneal of not larger than 600° C. Particularly, when the film thicknessis not larger than 300 Å, the film can be hardly crystallized. Also inthe case of the low temperature polysilicon, it becomes difficult tolower an OFF-state leak current at an OFF-state if the film thickness ofsilicon is not made thin, so that the thickness is desired to be as thinas possible. Although the silicon film with a thickness of not less than300 Å can be used as a driver circuit or the like by using an LDDstructure, it is preferable that the thickness is not larger than 300 Åin order to use the silicon film as a switching transistor for a pixel.

In the case where the amorphous silicon with a thickness of not largerthan 300 Å is crystallized, laser crystallization using a laser of awavelength of not larger than 400 nm, such as an excimer laser, iseffective. Crystallization is possible even for a film thickness of notlarger than 300 Å by crystallization using a laser, and the lasercrystallization is considerably used as a method of manufacturing a lowtemperature polysilicon TFT.

However, although crystallization by a laser is industrially possiblewhen the substrate is small, when the substrate becomes large,crystallization of the entire surface of the substrate by a laser takesan extremely long time so that laser crystallization is not appropriatefrom the industrial standpoint. Further, since there is no laser devicecapable of annealing a large substrate at once, the entire of thesubstrate is crystallized by repeating partial laser crystallization.Accordingly, irregularity of laser irradiation directly causesirregularity of characteristics of TFTs.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a thinfilm semiconductor device which combines only merits of an amorphoussilicon TFT and a polysilicon TFT by using a TFT of a low temperaturepolysilicon using laser crystallization or the like as a drivingcircuit, and an amorphous silicon transistor as a switching transistorfor a liquid crystal pixel, so that a thin film semiconductor unithaving switching characteristics of a small OFF-state current and anON/OFF ratio of not less than five figures is provided for the pixel,and a thin film semiconductor unit having high mobility is provided fora driver to integrate the driving circuit on a substrate.

In order to achieve the above object, according to the presentinvention, a thin film semiconductor device comprising a substratehaving an insulating surface, gate electrodes disposed on saidinsulating surface, gate insulating films disposed on upper portions ofsaid gate electrodes, and thin film semiconductors disposed on said gateinsulating films and including channel forming regions, source regionsand drain regions,

wherein one of said gate electrodes has a first area being contact withsaid substrate and a second area being contact with said gate insulatingfilm, said first area being larger than said second area, and

wherein a plurality of thin film semiconductor units are disposed onsaid substrate, said plurality of semiconductor units comprising:

a first thin film semiconductor unit including said thin filmsemiconductor of polycrystal, an insulating film covering an upperportion of said channel forming region in said polycrystal thin filmsemiconductor, impurity semiconductor films doped with trivalent orpentavalent impurities and covering said source region and said drainregion in said polycrystal thin film semiconductor, and conductive filmsdisposed on said impurity semiconductor films, said impuritysemiconductor films and said conductive films extending to saidinsulating film on said channel region, and being equal to each other inplane shape; and

a second thin film semiconductor unit including said thin filmsemiconductor of amorphous, an insulating film covering said channelregion in said amorphous thin film semiconductor, impurity semiconductorfilms doped with trivalent or pentavalent impurities and covering saidsource region and said drain region in said amorphous thin filmsemiconductor, and conductive films disposed on said impuritysemiconductor films, said impurity semiconductor films and saidconductive films extending to said insulating film on said channelregion, and being equal to each other in plane shape.

Further, according to the present invention, a thin film semiconductordevice comprising a substrate having an insulating surface, gateelectrodes disposed on said insulating surface, gate insulating filmsdisposed on upper portions of said gate electrodes, and thin filmsemiconductors disposed on said gate insulating films and includingchannel forming regions, source regions and drain regions,

wherein one of said gate electrodes has a first area being in contactwith said substrate and a second area being in contact with said gateinsulating film, said first area being larger than said second area,

wherein a plurality of thin film semiconductor units are disposed onsaid semiconductor, said plurality of semiconductor units comprising:

a first semiconductor unit including said thin film semiconductor ofpolycrystal, said channel forming region in said polycrystal thin filmsemiconductor having a thickness thinner than that of said source regionand said drain region in said polycrystal thin film semiconductor,impurity semiconductor films doped with trivalent or pentavalentimpurities and covering said source region and said drain region, andconductive films disposed on said impurity semiconductor films, saidimpurity semiconductor films and said conductive films being equal toeach other in plane shape; and

a second semiconductor unit including said thin film semiconductor ofamorphous, said channel forming region in said amorphous thin filmsemiconductor having a thickness thinner than that of said source regionand said drain region in said amorphous thin film semiconductor,impurity semiconductor films doped with trivalent or pentavalentimpurities and covering said source region and said drain region, andconductive films disposed on said impurity semiconductor films, saidimpurity semiconductor films and said conductive films being equal toeach other in plane shape.

Still further, a thin film semiconductor device comprising a substratehaving an insulating surface, gate electrodes disposed on saidinsulating surface, gate insulating films disposed on upper portions ofsaid gate electrodes, and thin film semiconductors disposed on said gateinsulating films and having channel forming regions, source regions anddrain regions,

wherein one of said gate electrodes has a first area being in contactwith said substrate and a second area being in contact with said gateinsulating film said first area being larger than said second area, and

wherein a plurality of thin film semiconductor units are disposed onsaid substrate, said plurality of semiconductor units comprising:

a first thin film semiconductor unit including said thin filmsemiconductor of polycrystal, an insulating film covering an upperportion of said channel region in said polycrystal thin filmsemiconductor, regions doped with trivalent or pentavalent impurities insaid source region and said drain region in said polycrystal thin filmsemiconductor, and conductive films disposed on said regions doped withsaid impurities, said conductive films extending to said insulating filmon said channel region; and

a second thin film semiconductor unit including said thin filmsemiconductor of amorphous, an insulating film covering an upper portionof said channel region in said amorphous thin film semiconductor,regions doped with trivalent or pentavalent impurities in said sourceregion and drain region in said amorphous thin film semiconductor, andconductive films disposed on said regions doped with said impurities,said conductive films extending to said insulating film on said channelregion.

Still further, a thin film semiconductor device comprising a substratehaving an insulating surface, gate electrodes disposed on saidinsulating surface, gate insulating films disposed on upper portions ofsaid gate electrodes, and thin film semiconductors disposed on said gateinsulating films and having channel forming regions, source regions anddrain regions,

wherein one of said gate electrodes has a first area being in contactwith said substrate and a second area being in contact with said gateinsulating film said first area being larger than said second area, and

wherein a plurality of thin film semiconductor units are disposed onsaid substrate, said plurality of semiconductor units comprising:

a first thin film semiconductor unit including said thin filmsemiconductor of polycrystal, said thin film semiconductor composed of achannel region, a drain region, a source region, a lightly doped sourceregion, and a lightly doped drain region, a first insulating filmcovering said channel region, said lightly doped source region, and saidlightly doped drain region, a second insulating region on said firstinsulating film and over said channel region, and regions doped withtrivalent or pentavalent impurities in said source region, said drainregion, said lightly doped source region, and lightly doped drainregion, the dose amount of said impurities in said source region andsaid drain region being larger than that of said impurities of saidlightly doped source region and said lightly doped drain region; and

a second thin film semiconductor unit including said thin filmsemiconductor of amorphous, an insulating film made of the same materialas said second insulating film, having the same thickness as said secondinsulating film, and covering an upper portion of said channel region insaid amorphous thin film semiconductor, regions doped with trivalent orpentavalent impurities in said source region and drain region in saidamorphous thin film semiconductor, and conductive films disposed on saidregions doped with said impurities, said conductive films extending tosaid insulating film on said channel region. Still further, a thin filmsemiconductor device comprising a substrate having an insulatingsurface, gate electrodes disposed on said insulating surface, gateinsulating films disposed on upper portions of said gate electrodes, andthin film semiconductors disposed on said gate insulating films andhaving channel forming regions, source regions and drain regions,

wherein one of said gate electrodes has a first area being contact withsaid substrate and a second area being contact with said gate insulatingfilm said first area being larger than said second area, and

wherein a plurality of thin film semiconductor units are disposed onsaid substrate, said plurality of semiconductor units comprising:

a first thin film semiconductor unit including said thin filmsemiconductor of polycrystal, said thin film semiconductor composed of achannel region, a drain region, a source region, a lightly doped sourceregion, and a lightly doped drain region, a first insulating filmcovering said channel region, a second insulating film on said lightlydoped source region, said lightly doped drain region, and said firstinsulating film, and regions doped with trivalent or pentavalentimpurities in said source region, said drain region, said lightly dopedsource region, and lightly doped drain region, the dose amount of saidimpurities in said source region and said drain region being larger thanthat of said impurities of said lightly doped source region and saidlightly doped drain region; and

a second thin film semiconductor unit including said thin filmsemiconductor of amorphous, an insulating film made of the same materialas said second insulating film, having the same thickness as said secondinsulating film, and covering an upper portion of said channel region insaid amorphous thin film semiconductor, regions doped with trivalent orpentavalent impurities in said source region and said drain region insaid amorphous thin film semiconductor, and conductive films disposed onsaid regions doped with said impurities, said conductive films extendingto said insulating film on said channel region.

Still further, a method of manufacturing a thin film semiconductordevice comprising a substrate having an insulating surface, gateelectrodes disposed on said insulating surface, gate insulating filmsdisposed on upper portions of said gate electrodes, and thin filmsemiconductors disposed on said gate insulating films and includingchannel forming regions, source regions and drain regions, said methodcomprising the steps of:

forming a gate electrode film of metal on said substrate;

forming a gate insulating film on said gate electrode film under a lowpressure;

forming an amorphous semiconductor thin film on said gate insulatingfilm under a low pressure without exposing said gate insulating film tothe atmosphere;

crystallizing a part of said amorphous semiconductor thin film withoutexposing said amorphous semiconductor thin film to the atmosphere;

forming an etching stopper insulating film on said semiconductor thinfilm without exposing said semiconductor thin film to the atmosphere;

taking out said substrate into the atmosphere;

forming an etching stopper insulating film on said channel formingregion;

shaping said thin film semiconductor and said gate insulating film intoa desired shape,

forming a semiconductor film containing trivalent or pentavalentimpurities;

forming a conductive film on said semiconductor film; and

shaping said semiconductor film containing said impurities and saidconductive film into the same plane shape;

whereby a thin film semiconductor unit in which said channel formingregion is made of polycrystal, and a thin film semiconductor unit inwhich said channel forming region is made of amorphous, are formed onsaid substrate.

Still further, a method of manufacturing a thin film semiconductordevice comprising a substrate having an insulating surface, a gateelectrode disposed on said insulating surface, a gate insulating filmdisposed on an upper portion of said gate electrode, and a thin filmsemiconductor disposed on said gate insulating film and including achannel forming region, a source region and a drain region, said methodcomprising the steps of:

forming a gate electrode film of metal on said substrate;

forming a gate insulating film on said gate electrode film under a lowpressure;

forming an amorphous semiconductor thin film on said gate insulatingfilm under a low pressure without exposing said gate insulating film tothe atmosphere;

crystallizing a part of said semiconductor thin film without exposingsaid semiconductor thin film to the atmosphere; forming a semiconductorfilm containing trivalent or pentavalent impurities on saidsemiconductor thin film without exposing said semiconductor thin film tothe atmosphere;

forming a conductive film on said semiconductor film containing saidimpurities without exposing said semiconductor film to the atmosphere;

taking out said substrate into the atmosphere; and

shaping said conductive film, said semiconductor film containing saidimpurities, and a part of said semiconductor thin film into a desiredshape;

whereby a thin film semiconductor unit in which said channel formingregion is made of polycrystal, and a thin film semiconductor unit inwhich said channel forming region is made of amorphous, are formed onsaid substrate.

Still further, a method of manufacturing a thin film semiconductordevice comprising a substrate having an insulating surface, a gateelectrode disposed on said insulating surface, a gate insulating filmdisposed on an upper portion of said gate electrode, and a thin filmsemiconductor disposed on said gate insulating film and including achannel forming region, a source region and a drain region, said methodcomprising the steps of:

forming a gate electrode film of metal on said substrate;

forming a gate insulating film on said gate electrode film under a lowpressure;

forming an amorphous semiconductor thin film on said gate insulatingfilm under a low pressure without exposing said gate insulating film tothe atmosphere;

crystallizing a part of said semiconductor thin film without exposingsaid semiconductor thin film to the atmosphere;

forming an etching stopper insulating film on said semiconductor thinfilm without exposing said semiconductor thin film to the atmosphere;

taking out said substrate into the atmosphere;

forming an etching stopper insulating film on said channel formingregion;

shaping said thin film semiconductor and said gate insulating film intoa desired shape;

forming said source region and said drain region by doping of trivalentor pentavalent impurities; and

forming a conductive film on said source region and said drain regionsuch that at least part of said conductive film is brought into contactwith said source region and said drain region;

whereby a thin film semiconductor unit in which said channel formingregion is made of polycrystal, and a thin film semiconductor unit inwhich said channel forming region is made of amorphous, are formed onsaid substrate.

Still further, a method of manufacturing a thin film semiconductordevice comprising a substrate having an insulating surface, a gateelectrode disposed on said insulating surface, a gate insulating filmdisposed on an upper portion of said gate electrode, and a thin filmsemiconductor disposed on said gate insulating film and including achannel forming region, a source region and a drain region, said methodcomprising the steps of:

forming a gate electrode film of metal on said substrate;

forming a gate insulating film on said gate electrode film under a lowpressure;

forming an amorphous semiconductor thin film on said gate insulatingfilm under a low pressure without exposing said gate insulating film tothe atmosphere;

crystallizing a part of said semiconductor thin film without exposingsaid semiconductor thin film to the atmosphere;

forming a first insulating film on said semiconductor thin film withoutexposing said semiconductor thin film to the atmosphere;

taking out said substrate into the atmosphere;

shaping said first insulating film so that said first insulating filmhaving an area larger than said gate electrode remains on saidcrystallized semiconductor thin film, and said first insulating filmdoes not remain on said semiconductor thin film not crystallized;

forming a second insulating film having an area smaller than said gateelectrode on said first insulating film on said crystallizedsemiconductor thin film and on said semiconductor thin film notcrystallized;

forming said source region, said drain region, said lightly doped sourceregion, and said lightly doped drain region in said crystallizedsemiconductor thin film, and forming said source region and said drainregion in said semiconductor thin film not crystallized, by doping oftrivalent or pentavalent impurities; and

recrystallizing only said crystallized semiconductor thin film;

whereby a thin film semiconductor unit in which said channel formingregion is made of polycrystal, and a thin film semiconductor unit inwhich said channel forming region is made of amorphous, are formed onsaid substrate.

Still further, a method of manufacturing a thin film semiconductordevice comprising a substrate having an insulating surface, a gateelectrode disposed on said insulating surface, a gate insulating filmdisposed on an upper portion of said gate electrode, and a thin filmsemiconductor disposed on said gate insulating film and including achannel forming region, a source region and a drain region, said methodcomprising the steps of:

forming a gate electrode film of metal on said substrate;

forming a gate insulating film on said gate electrode film under a lowpressure;

forming an amorphous semiconductor thin film on said gate insulatingfilm under a low pressure without exposing said gate insulating film tothe atmosphere;

crystallizing a part of said semiconductor thin film without exposingsaid semiconductor thin film to the atmosphere;

forming a first insulating film on said semiconductor thin film withoutexposing said semiconductor thin film to the atmosphere;

taking out said substrate into the atmosphere;

shaping said first insulating film so that said first insulating filmhaving an area smaller than said gate electrode remains on saidcrystallized semiconductor thin film, and said first insulating filmcovers all of said channel forming region, said drain region, and saidsource region on said semiconductor thin film not crystallized;

conducting light doping of trivalent or pentavalent impurities so that aregion not covered with said first insulating film is lightly doped, andsaid semiconductor thin film not crystallized is hardly doped;

recrystallizing only said crystallized semiconductor thin film;

forming a second insulating film having an area larger than said gateelectrode on said first insulating film on said crystallizedsemiconductor thin film and forming said second insulating film havingan area smaller than said gate electrode on said semiconductor thin filmnot crystallized;

forming said source region, said drain region, said lightly doped sourceregion, and said lightly doped drain region in said crystallizedsemiconductor thin film, and forming said source region and said drainregion in said semiconductor thin film not crystallized, by doping oftrivalent or pentavalent impurities; and

recrystallizing only said crystallized semiconductor thin film;

whereby a thin film semiconductor unit in which said channel formingregion is made of polycrystal, and a thin film semiconductor unit inwhich said channel forming region is made of amorphous, are formed onsaid substrate.

In order to form polycrystal silicon (polysilicon) transistors andnoncrystal silicon (amorphous silicon) transistors on the samesubstrate, a bottom gate type and reverse stagger type structure isadopted in the present invention. The reason is as follows. That is,particularly in the case of an amorphous silicon TFT, since an interfacebetween a gate insulating film and an amorphous silicon as an activelayer becomes a channel, it is continuously formed and the gateinsulating film is formed at a temperature as high as possible to form adense film. Thus, if the amorphous silicon is formed before the gateinsulating film is formed, hydrogen in the amorphous silicon is drawnoff, so that the function as a semiconductor is lost. Accordingly, thegate insulating film is formed at a high temperature in advance, andthen the amorphous silicon as an active layer is formed.

As a gate electrode, any film may be used as long as it is a metal film.In the present invention, Al, Ta, Cr, Mo, or alloy containing the formermetal as a main component is used. As a film forming method, magnetronsputtering, electron beam evaporation, or resistor heating evaporationis used. In view of heat resistance at subsequent laser crystallization,and to prevent short circuit between a source or drain and a gate, it iseffective to cover the gate electrode with a dense oxide film by anodicoxidation of the metal as mentioned above. The shape of the gateelectrode greatly affects subsequent processes. Especially, since thefilm thickness of silicon is thin, the edge of the gate electrode isrequired to be taper-shaped, and it is necessary to form such a tapershape that an area of the gate electrode being contact with the gateinsulating film is smaller than an area of the gate electrode beingcontact with the substrate.

In the case where Al is used for the metal film, by application of heatof not lower than 200° C. at a subsequent film formation of the gateinsulating film or the like, hillocks are sometimes produced. Thegeneration of hillocks can be prevented by forming a film of Al mixedwith Si or Sc of 0.1 to 2% as impurities, or by performing high vacuumaluminum film formation under a pressure of not larger than 10⁻⁷ Torr byevacuation before formation of an Al film. Although the film thicknessis also dependent on a resistance value, it is necessary to make such athickness that the sheet resistance becomes about 10 Ω/• or less.

After forming the gate electrode of only a metal film or a metal filmcovered with an anodic oxidation film, an insulating film as a gateinsulating film is formed. As the insulating film, a single layer ormultilayer of silicon oxide, silicon nitride, silicon nitride oxide isformed.

As a film forming method, reactive sputtering, plasma CVD, low pressureCVD, or the like is used. Since the insulating film is used as the gateinsulating film, it is necessary to make the film dense by forming thefilm at a temperature as high as possible.

As the gate insulating film used for an amorphous TFT for switching aliquid crystal pixel, it has been found by experiment that a nitridefilm is superior in coordination to an oxide film. Accordingly, as thegate insulating film, a single layer of nitride film or multilayerstructure in which only a layer being contact with the amorphous siliconis a nitride film, is good as the structure. Although the film thicknessof the gate insulating film is dependent on the dielectric constant, thethickness of about 1,000 to 3,000 Å is required.

After forming the gate insulating film, amorphous silicon to be formedinto an active layer is formed without exposing the surface thereof tothe atmosphere. As a method of forming the amorphous silicon, plasma CVDmethod, sputtering method, low pressure CVD method, or the like is used.Although the amorphous silicon can be formed in the same reactivechamber as that where the gate insulating film has been formed, in viewof the contamination of an interface between the gate insulating film tobe made into the channel formation region and the amorphous siliconfilm, it is desirable to form the amorphous silicon in a differentreactive chamber. Although it is preferable that the film thickness isas thin as possible, the thickness of not smaller than 100 Å is requiredin view of subsequent crystallization, and in view of OFF-state leakcurrent of a TFT, the thickness is preferably not larger than 300 Å.Accordingly, the thickness is selected within the range of 100 to 300 Å.

After forming the amorphous silicon, to form a driving circuit or thelike on the substrate, it is necessary to crystallize only a portionwhere polysilicon is subsequently required. In the substrate where theamorphous silicon has been formed, portions where driver circuits fordriving liquid crystal are two portions, in the lengthwise and widthwisedirections, of the end of the substrate. In order to crystallize thoseportions, laser crystallization by scanning only those portions with alaser beam, or lamp annealing to only those portions is carried out.

In the case of crystallization by a laser, since the amorphous siliconfilm is thin, in order to make the influence to the gate insulating filmand the gate electrode as the under layer of the amorphous silicon filmas little as possible, annealing is carried out by an excimer laserhaving a wavelength of 308 nm or 254 nm. Alternatively, lamp heatingusing the principle of rapid thermal anneal is preferable.

In order to carry out partial crystallization, in the case of a laser,the shape of a laser beam is made linear by an optical system, and theportion in the lengthwise direction of the substrate is irradiated atonce or in a divided manner to be crystallized. Then the substrate isrotated by 90°, and the portion in the widthwise direction of thesubstrate is irradiated at once or in a divided manner to becrystallized. Although it is possible to scan the optical system of thelaser by using a galvanometer or the like without rotating thesubstrate, since the optical system taking the aberration intoconsideration becomes complicated because of the short wavelength, it ispreferable to fix the laser beam and to move the substrate instead.

In the case where partial crystallization is made by lamp annealing, theportion on the substrate where crystallization is not carried out iscovered with a mask of metal or ceramics, and the lamp irradiates theentire surface of the substrate. According to experiments by the presentinventor, when a metal mask was repeatedly used many times, thereoccurred such a problem that the mask was bent. Thus, ceramics was usedas the mask. However, when a mask is not continuously used, a metal maskmay be used.

In the case of crystallization, it is preferable to carry out thecrystallization in vacuum or an inert gas than in the atmosphere. In thepresent invention, after forming the amorphous silicon, the substrate istransferred into a low pressure chamber for crystallization withoutexposing the substrate to the atmosphere, and crystallization is carriedout in the vacuum state.

After forming the amorphous silicon and the polysilicon on the samesubstrate, source and drain regions are formed. There are some methodsof forming the regions.

According to a first method, an insulating film for an etching stopperis provided on an upper portion of a channel forming region. Theinsulating film for the etching stopper is formed on the entire surfaceof the substrate where partial crystallization is completed. The film isalso formed on the amorphous silicon and the crystallized polysiliconwithout exposing the substrate to the atmosphere. As the material of thefilm, any material may be used as long as the insulating film has a highselective ratio of etching to the silicon, and silicon nitride, siliconoxide, silicon nitride oxide may be used.

When the insulating film for the etching stopper is formed so that anarea thereof is smaller than an area of an upper portion of the gateelectrode, the area of the etching stopper is equivalent to the area ofthe channel. After forming the insulating film, the surfaces of theamorphous silicon and polysilicon, which becomes the source and drainregions, are exposed, and the channel forming portion is covered withthe etching stopper.

Next, an amorphous impurity semiconductor layer which is doped withtrivalent or pentavalent impurities such as phosphorous and boron, isformed on the entire surface, and a conductive layer as a wiring layeris continuously formed. Thereafter, the conductive layer and theimpurity semiconductor layer are formed using the same photo mask sothat they extends to an upper portion of the etching stopper and coverthe source region and drain region. Thus, a silicon TFT and apolysilicon TFT are completed.

Although the amorphous silicon TFT is almost completed by the processdescribed above, in the case of the polysilicon TFT, since the impuritysemiconductor being contact with the source and drain regions is notcrystalline semiconductor, if crystallization like the former partialcrystallization is carried out after forming the impurity semiconductorlayer, more completed TFT is made. When the present invention is appliedto the AMLCD, although the amorphous silicon TFTs are only N-channelTFTs, in the case of the polysilicon TFTs, since complementarytransistors are required to be formed, the impurity semiconductor filmdoped with boron while unnecessary portions are masked, is formed sothat P-channel TFTs are formed.

According to a second method, a part of channel formation region isetched. After completing partial crystallization, a semiconductor layerdoped with impurities is formed without exposing the substrate to theatmosphere, and thereafter a conductive film as a wiring layer is formedwithout exposing the substrate to the atmosphere.

Thereafter, the conductive film and the semiconductor layer doped withimpurities are etched into the same plane shape to form a wiring region.At that time, a portion of the silicon thin film as the active layer onthe gate electrode is etched, so that a channel forming region isformed. Accordingly, the film thickness of the active layer of thechannel forming region is thinner than the source and drain regions.

Although the amorphous silicon TFT is almost completed by the methoddescribed above, in the case of the polysilicon TFT, since the impuritysemiconductor being contact with the source and drain regions is notcrystalline semiconductor, if crystallization like the former partialcrystallization is carried out after forming the impurity semiconductorlayer, more completed TFT is made. Although the amorphous silicon TFTsare only N-channel TFTs, in the case of the polysilicon TFTs, sincecomplementary transistors are required to be formed, the impuritysemiconductor film doped with boron while unnecessary portions aremasked, is formed, so that P-channel TFTs are formed.

According to a third method, instead of forming an impuritysemiconductor layer, plasma doping, ion implantation, ion doping, or thelike is carried out to source and drain regions, so that the source anddrain region can be formed.

Further, the polysilicon TFT can be made into an LDD structure. Byforming a lightly doped drain region and a lightly doped source regionhaving dose amounts of impurity smaller than a source region and a drainregion, the LDD structure can be formed.

In the thus formed polysilicon TFTs and amorphous silicon TFTs on thesame substrate, when the polysilicon TFTs are used as a driving circuit,the circuit having high frequency response characteristics can beformed, and at the same time, when the amorphous silicon TFTs are usedfor pixels, switching elements having high ON/OFF ratio and smallOFF-state leak current can be formed. Accordingly, high quality liquidcrystal display is possible. Further, since circuits can be formed onthe same substrate of the cheap normal glass, the present invention hasgreat advantages in technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) and 1(B) are views showing transistors of the presentinvention;

FIGS. 2(A) and 2(B) are views showing transistors of the presentinvention;

FIGS. 3(A) and 3(B) are views showing transistors of the presentinvention;

FIGS. 4(A) and 4(B) are views showing an apparatus for practicing thepresent invention;

FIGS. 5(A) to 5(C) are views showing steps of laser crystallization ofthe present invention; and

FIGS. 6(A) and 6(B) are views showing an apparatus for practicing thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

A manufacturing method of forming polysilicon TFTs and amorphous siliconTFTs on the same substrate using the present invention will bedescribed.

FIG. 1(A) shows a first embodiment of the present invention. A substrate1001 has an insulating surface and it is made of an insulator such assoda glass, borosilicate glass or quartz, one coated with a PSG film,silicon oxide film, or silicon nitride film, or the like. Thisembodiment uses a substrate made of 7059 glass produced by Corning Co.,Ltd. as the borosilicate glass, coated with a silicon oxide film bysputtering.

After forming a conductive film for a gate electrode 1002 on thesubstrate 1001, it is formed into a desired shape by using aphotolithographic technique to obtain the gate electrodes 1002. At thisformation, since a subsequently formed active layer is thin to be 100 to300 Å, the gate electrode must be formed so that the cross sectionthereof is taper-shaped as shown in the drawing. As the material of thegate electrode 1002, there is used metal such as tantalum, chromium,molybdenum, or aluminum. If possible, it is preferable to use such metalthat the surface thereof can be subjected to anodic oxidization using ananodic oxidation technique. In this embodiment, after forming analuminum film, a gate AO film 1003 is formed by the anodic oxidation.

Although the anodic oxidation film is not necessarily required, in viewof heat resistance in subsequent laser crystallization or the like andto prevent a leak between a gate electrode and a drain or source of theamorphous silicon TFT, it is preferable to carry out the anodicoxidation.

When an aluminum film is formed, since a subsequent heat process is aptto cause hillocks so that the leak between the gate electrode and thedrain or source is easily generated, the film formation must beinventively conducted.

When a pure aluminum film is formed under a background pressure bysufficient evacuation before the film is formed, the hillocks are hardto be generated. Thus, before the film is formed, if the film formationis carried out after the pressure is made not larger than 10⁻⁷ Torr, thegeneration of hillocks can be greatly reduced. Also, if silicon orscandium of about 0.1 to 2% is mixed into aluminum, the generation ofhillocks can be greatly reduced.

In this embodiment, the aluminum film is formed by using an ultra highvacuum sputtering apparatus to prevent the hillocks from beinggenerated. In this sputtering apparatus, all inner walls of a filmforming chamber are subjected to electrolytic polishing, and a turbomolecular pump and a cryopump are provided to make the backgroundpressure not larger than 10⁻⁷ Torr, so that the background pressure canbe reduced to about 5×10⁻⁸ Torr. Further, an argon gas to be introducedis used after refining at a use point of the apparatus. Although thefilm thickness of the gate electrode 1002 is about 1,000 to 3,000 Å, thefilm thickness of 500 Å is sufficient for aluminum which requires such afilm thickness that a sheet resistance is not larger than 1 Ω/•.However, in order to further reduce the resistance, the film thicknessin this embodiment is 1,500 Å.

After forming the gate electrode 1002 and the gate AO film 1003, a gateinsulating film 1004 is formed. As the gate insulating film 1004, thereis used a single layer or a multilayer of a silicon nitride film,silicon oxide film, or silicon nitride oxide film. As a film formingmethod, a plasma CVD method is used. In this embodiment, the siliconnitride film is formed by a parallel flat plate plasma CVD method. Thefilm thickness of the gate insulating film 1004 is comparable to orlarger than that of the gate electrode 1002. In this embodiment, thesilicon nitride film of 3,000 Å is formed.

After forming the gate insulating film 1004, an amorphous silicon filmas an active layer for forming a channel forming region and source anddrain regions is formed. Unless the amorphous silicon film is formedwithout exposing it to the atmosphere after the formation of the gateinsulating film 1004, a number of interface states are generated in theinterface between the gate insulating film 1004 and the active layer sothat a superior transistor can not be formed. Accordingly, continuousfilm formation of forming a film without breaking a vacuum is important.

As methods of forming the active layer, there are a plasma CVD method, alow pressure CVD method, a photo CVD method, a sputtering method, andthe like. In this embodiment, the sputtering method is used. In thesputtering, reactive sputtering by a mixture gas of argon and hydrogenis carried out, so that the amorphous silicon film of 100 to 300 Å isformed as the active layer. This is because when the thickness is notlarger than 100 Å, crystallization by a laser or the like is hard, andwhen the thickness is not less than 300 Å, an OFF-state leak current ofthe TFT becomes large. In this embodiment, the amorphous silicon film ofabout 200 Å is formed by the reactive sputtering.

After forming the active layer, portions where polysilicon TFTs arerequired for subsequent formation of driving circuits or the like arecrystallized. The active layer is partially crystallized by irradiationof an excimer laser having a wavelength of not larger than 400 nm suchas an ArF or XeCl excimer laser, so that a polysilicon active layer 1010and an amorphous active layer 1011 not crystallized are formed. Since ithas been found by experiment that when laser crystallization is carriedout in a vacuum or hydrogen gas, superior crystal can be obtained.Accordingly, in this embodiment, crystallization is carried out in thevacuum of 10⁻⁵ Torr by the irradiation of the KrF excimer laser(wavelength 248 nm, pulse width 20 nsec). The energy density of thelaser is 300 mJ/cm² and ten shots are carried out.

Although heating of the substrate 1001 at the laser crystallization iseffective for obtaining superior crystallization, if the substrate isheated up to an excessively high temperature, hydrogen in the amorphousactive layer 1011 is drawn off. Accordingly, the crystallization ispreferably carried out at a temperature of not larger than 300° C. Inthis embodiment, the crystallization is carried out in the state wherethe substrate 1001 is heated up to 150° C.

After the crystallization is completed, an insulating film as an etchingstopper 1020 is formed without exposing the substrate to the atmosphere.Any insulating films may be used as long as they assure a selectiveratio with respect to silicon at subsequent etching. Silicon oxide,silicon nitride, silicon nitride oxide, or the like is used. In thisembodiment, the silicon nitride film of 1,000 Å by the plasma CVD methodis formed.

After forming the insulating film for the etching stopper 1020, thesubstrate 1001 is taken out into the atmosphere. With respect to thesubstrate 1001 taken out into the atmosphere, the insulating film forthe etching stopper 1020 is etched by a photolithographic technique toform the etching stopper 1020. This etching stopper 1020 determines achannel forming region of the TFT. As shown in the drawing, the etchingstopper is formed so that the width thereof is narrower than that of thegate electrode 1002. Next, by using the photolithographic technique, theactive layers 1010 and 1011, and the gate insulating film 1004 areformed into desired shapes. In this embodiment, although the activelayers 1010 and 1011 and the gate insulating film 1004 are formed intothe same shape, it is absolutely unnecessary to form them into the sameshape.

Next, an impurity semiconductor layer for source and drain regions and awiring layer are formed. The impurity semiconductor layer is formed bythe plasma CVD method or low pressure CVD method using a mixture gas ofa source gas for silicon and a trivalent or pentavalent impurity gassuch as boron or phosphorous. Thereafter, a conductive film for thewiring layer is formed. As the conductive film, a metal film or atransparent conductive oxide film of ITO or the like is used. Afterforming the impurity semiconductor layer and the wiring layer, by usingthe photolithographic technique, a source region 1030, a drain region1031, a source electrode 1040, and a drain electrode 1041 are formed sothat the state shown in FIG. 1(A) is obtained.

The left side in FIG. 1(A) is the polysilicon TFT, and the right sidethereof is the amorphous silicon TFT. Although they are formed to beadjacent to each other in the drawing, the polysilicon TFT ispractically formed on a portion of the substrate where a driving circuitis formed, and the amorphous TFT is formed on a portion thereof where aswitching element for a pixel is formed.

FIG. 1(B) shows a modified embodiment almost equal to FIG. 1(A) in theprocess and structure. Reference numerals in FIG. 1(B) correspond tothose in FIG. 1(A). The only difference is that a gate insulating filmis formed of two layers, that is, a first gate insulating film 1004 anda second gate insulating film 1005 are provided.

Although it has been found by experiment that in the amorphous siliconTFT, a silicon nitride film is preferable as the gate insulating filmbeing contact with the channel forming region made of amorphous silicon.However, since the relative dielectric constant of the silicon nitridefilm is about two times as large as that of the silicon oxide film, thethickness thereof becomes thick. Thus, when only the portion beingcontact with the amorphous silicon is formed of the silicon nitridefilm, and other portion is formed of the silicon oxide film, the filmthickness of the entire gate insulating film can be made thin.

In this modified embodiment, the silicon oxide film of 1,000 Å is formedas the first gate insulating film 1004 by the plasma CVD method using anorganic silane, and the silicon nitride film of 50 Å is formed as thesecond gate insulating film 1005 by the plasma CVD method using amixture gas of silane, ammonium and nitrogen

Second Embodiment

A manufacturing method of forming polysilicon TFTs and amorphous siliconTFTs on the same substrate by using the present invention will bedescribed.

FIG. 2(A) shows a second embodiment of the present invention. Asubstrate 2001 has an insulating surface and it is made of an insulatorsuch as soda glass, borosilicate glass or quartz, one having a coatingfilm of PSG, silicon oxide or silicon nitride on the insulator, or thelike. This embodiment uses a substrate made of 1737 glass produced byCorning Co., Ltd. as the borosilicate glass, coated with a siliconnitride film by the low pressure CVD.

After forming a conductive film for a gate electrode 2002 on thesubstrate 2001, it is formed into a desired shape by using aphotolithographic technique to obtain the gate electrodes 2002. At thisformation, since a subsequently formed active layer is thin to be 100 to300 Å, the gate electrode must be formed so that the cross sectionthereof is taper-shaped as shown in the drawing. As the material of thegate electrode 2002, there is used metal such as tantalum, chromium,molybdenum, or aluminum. If possible, it is preferable to use such metalthat the surface thereof can be subjected to anodic oxidation using ananodic oxidation technique. In this embodiment, after forming a tantalumfilm, a gate AO film 2003 is formed by the anodic oxidation.

Although the anodic oxidation film is not necessarily required, in viewof heat resistance in subsequent laser crystallization or the like andto prevent a leak between a gate electrode and a drain or source of theamorphous silicon TFT, it is preferable to carry out the anodicoxidation.

Although the film thickness of the gate electrode 2002 is about 1,000 to3,000 Å, the thickness of 1,500 Å is sufficient for tantalum whichrequires such a film thickness that a sheet resistance is not largerthan 1 Ω/□. However, in order to further reduce the resistance, the filmthickness in this embodiment is 2,000 Å.

After forming the gate electrode 2002 and the gate AO film 2003, a gateinsulating film 2004 is formed. As the gate insulating film 2004, thereis used a single layer or a multilayer of a silicon nitride film,silicon oxide film, or silicon nitride oxide film. As a film formingmethod, a plasma CVD film is used. In this embodiment, the siliconnitride film is formed by a parallel flat plate plasma CVD method. Thefilm thickness of the gate insulating film is comparable to or largerthan that of the gate electrode 2002. In this embodiment, the siliconnitride film of 3,000 Å is formed.

After forming the gate insulating film 2004, an amorphous silicon filmas an active layer for forming a channel forming region and source anddrain regions is formed. Unless the amorphous silicon film is formedwithout exposing it to the atmosphere after the formation of the gateinsulating film 2004, a number of interface states are generated in theinterface between the gate insulating film 2004 and the active layer sothat a superior transistor can not be formed. Accordingly, continuousfilm formation of forming a film without breaking a vacuum is important.

As methods of forming the active layer, there are a plasma CVD method, alow pressure CVD method, a photo CVD method, a sputtering method, andthe like. In this embodiment, the plasma CVD method is used. The plasmaCVD method is carried out using a silane gas to form the amorphoussilicon layer with a thickness of 100 to 300 Å as the active layer. Thisis because when the thickness is not larger than 100 Å, crystallizationby a laser or the like is hard, and when the thickness is not less than300 Å, an OFF-state leak current of the TFT becomes large. In thisembodiment, the amorphous silicon film of about 200 Å is formed by theplasma CVD method.

After forming the active layer, portions where polysilicon TFTs arerequired for subsequent formation of driving circuits or the like arecrystallized. The active layer is partially crystallized by irradiationof an excimer laser having a wavelength of not larger than 400 nm suchas an ArF or KrF excimer laser, so that a polysilicon active layer 2010and an amorphous active layer 2011 not crystallized are formed. Since ithas been found by experiment that when laser crystallization is carriedout in a vacuum or hydrogen gas, superior crystal can be obtained.Accordingly, in this embodiment, crystallization is carried out in thehydrogen gas of 10 Torr by the irradiation of the XeCl excimer laser(wavelength 308 nm, pulse width 30 nsec). The energy density of thelaser is 250 mJ/cm² and fifteen shots are carried out.

Although heating of the substrate 2001 at the laser crystallization iseffective for obtaining superior crystallization, if the substrate isheated up to an excessively high temperature, hydrogen in the amorphousactive layer 2011 is drawn off. Accordingly, the crystallization ispreferably to carry out at a temperature of not larger than 300° C. Inthis embodiment, the crystallization is carried out in the state wherethe substrate 2001 is heated up to 150° C.

After the crystallization is completed, an impurity semiconductor layerfor source and drain regions and a wiring layer are formed withoutexposing the substrate to the atmosphere. The impurity semiconductorlayer is formed by the plasma CVD method or low pressure CVD methodusing a mixture gas of a source gas for silicon and a trivalent orpentavalent impurity gas such as boron or phosphorous.

Thereafter, a conductive film for the wiring layer is formed. As theconductive film, a metal film or a transparent conductive oxide film ofITO or the like is used. After forming the impurity semiconductor layerand the wiring layer, by using the photolithographic technique, a sourceregion 2030, a drain region 2031, a source electrode 2040, and a drainelectrode 2041 are formed.

A channel forming region is formed by etching a part of the active layerwhen the source and drain are formed. Then, a polysilicon channel region2050 and an amorphous channel region 2051 are formed. The thickness ofthe active layer in the respective channel regions is slightly thinnerthan the source region 2030 and the drain region 2031 since the activelayer is slightly etched when the source and drain are formed. In thisway, the state as shown in FIG. 2(A) is obtained.

The left side in FIG. 2(A) is the polysilicon TFT, and the right sidethereof is the amorphous silicon TFT. Although they are formed to beadjacent to each other in the drawing, the polysilicon TFT ispractically formed on a portion of the substrate where a driving circuitis formed, and the amorphous TFT is formed on a portion thereof where aswitching element for a pixel is formed.

FIG. 2(B) shows a modified embodiment almost equal to FIG. 2(A) in theprocess and structure. Reference numerals in FIG. 2(B) correspond tothose in FIG. 2(A). The only difference is that a gate insulating filmis formed of two layers, that is, a first gate insulating film 2004 anda second gate insulating film 2005 are provided.

Although it has been found by experiment that in the amorphous siliconTFT, a silicon nitride film is preferable as the gate insulating filmbeing contact with a channel forming region made of amorphous silicon.However, since the relative dielectric constant of the silicon nitridefilm is about as two time as large as that of the silicon oxide film,the thickness thereof becomes thick. Thus, when only the portion beingcontact with the amorphous silicon is formed of the silicon nitridefilm, and other portion is formed of the silicon oxide film, the filmthickness of the entire gate insulating film can be made thin.

In this modified embodiment, the silicon oxide film of 1,000 Å is formedas the first gate insulating film 2004 by the reactive sputtering methodusing only an oxygen gas, and the silicon nitride film of 50 Å is formedas the second gate insulating film 2005 by the photo CVD method using amixture gas of silane, ammonium and nitrogen.

Third Embodiment

A manufacturing method of forming polysilicon TFTs and amorphous siliconTFTs on the same substrate by using the present invention will bedescribed.

FIG. 3(A) shows a third embodiment of the present invention. A substrate3001 has an insulating surface and it is made of an insulator such assoda glass, borosilicate glass or quartz, one coated with a PSG film,silicon oxide film or silicon nitride film, or the like. This embodimentuses a substrate of 7059 glass produced by Corning Co., Ltd. as theborosilicate glass, coated with a silicon oxide film by sputtering.

After forming a conductive film for a gate electrode 3002 on thesubstrate 3001, it is formed into a desired shape by using aphotolithographic technique to obtain the gate electrodes 3002. At thisformation, since a subsequently formed active layer is thin to be 100 to300 Å, the gate electrode must be formed so that the cross sectionthereof is taper-shaped as shown in the drawing. As the material of thegate electrode 3002, there is used metal such as tantalum, chromium,molybdenum, or aluminum. If possible, it is preferable to use such metalthat the surface thereof can be subjected to anodic oxidation using ananodic oxidation technique. In this embodiment, after forming analuminum film, a gate AO film 3003 is formed by the anodic oxidation.

Although the anodic oxidation film is not necessarily required, in viewof heat resistance in a subsequent laser crystallization or the like andto prevent a leak between a gate electrode and a drain or source of theamorphous silicon TFT, it is preferable to carry out the anodicoxidation.

When an aluminum film is formed, since a subsequent heat process is aptto cause hillocks so that the leak between the gate electrode and thedrain or source is generated, the film formation must be inventivelyconducted.

When a pure aluminum film is formed under a background pressure bysufficient evacuation, the hillocks are hard to be generated. Thus, ifthe film formation is carried out after the pressure is made not largerthan 10⁻⁷ Torr, the generation of hillocks can be greatly reduced. Also,if silicon or scandium of 0.1 to 2% is mixed into aluminum, thegeneration of hillocks can be greatly reduced.

In this embodiment, the aluminum film is formed by using an ultra highvacuum sputtering apparatus to prevent the hillocks from beinggenerated. In this sputtering apparatus, all inner walls of a filmforming chamber are subjected to electrolytic polishing, and a turbomolecular pump and a cryopump are provided to make the backgroundpressure not larger than 10⁻⁷ Torr, so that the background pressure canbe reduced to about 5×10⁻⁸ Torr. Further, an argon gas to be introducedis used after refining at a use point of the apparatus. Although thefilm thickness of the gate electrode 3002 is about 1,000 to 3,000 Å, thefilm thickness of 500 Å is sufficient for aluminum which requires such afilm thickness that a sheet resistance is not larger than 1 Ω/•.However, in order to further reduce the resistance, the thickness inthis embodiment is 1,500 Å.

After forming the gate electrode 3002 and the gate AO film 3003, a gateinsulating film 3004 is formed. As the gate insulating film 3004, thereis used a single layer or a multilayer of a silicon nitride film,silicon oxide film, or silicon nitride oxide film. As a film formingmethod, a plasma CVD method is used. In this embodiment, the siliconnitride film is formed by a parallel flat plate plasma CVD method. Thefilm thickness of the gate insulating film 3004 is comparable to orlarger than that of the gate electrode 3002. In this embodiment, thesilicon nitride film of 3,000 Å is formed.

After forming the gate insulating film 3004, an amorphous silicon filmas an active layer for forming a channel forming region and sourceregion and drain regions is formed. Unless the amorphous silicon film isformed without exposing it to the atmosphere after the formation of thegate insulating film 3004, a number of interface states are generated inthe interface between the gate insulating film 3004 and the active layerso that a superior transistor can not be formed.

Accordingly, continuous film formation of forming a film withoutbreaking a vacuum is important.

As methods of forming the active layer, there are a plasma CVD method, alow pressure CVD method, a photo CVD method, a sputtering method, andthe like. In this embodiment, the sputtering method is used. In thesputtering, reactive sputtering by a mixture gas of argon and hydrogenis carried out, so that the amorphous silicon film with a thickness of100 to 300 Å is formed as the active layer. This is because when thethickness is not larger than 100 Å, crystallization by a laser or thelike is hard, and when the thickness is not less than 300 Å, anOFF-state leak current of the TFT becomes large. In this embodiment, theamorphous silicon of about 200 Å is formed by the reactive sputtering.

After forming the active layer, portions where polysilicon TFTs arerequired for subsequent formation of driving circuits or the like arecrystallized. The active layer is partially crystallized by irradiationof an excimer laser having a wavelength of not larger than 400 nm suchas an ArF or XeCl excimer laser, so that a polysilicon active layer 3010and an amorphous active layer 3011 not crystallized are formed. Since ithas been found by experiment that when laser crystallization is carriedout in a vacuum or hydrogen, superior crystal can be obtainedaccordingly, in this embodiment, crystallization is carried out in thevacuum of 10⁻⁵ Torr by irradiation of the KrF excimer laser (wavelength248 nm, pulse width 20 nsec). The energy density of the laser is 300mJ/cm² and ten shots are carried out.

Although heating of the substrate 3001 at the laser crystallization iseffective for obtaining superior crystallization, if the substrate isheated up to an excessively high temperature, hydrogen in the amorphousactive layer 3011 is drawn off. Accordingly, the crystallization ispreferably carried out at a temperature of not larger than 300° C. Inthis embodiment, the crystallization is carried out in the state wherethe substrate 3001 is heated up to 150° C.

After crystallization is completed, an insulating film as an etchingstopper 3020 is formed without exposing the substrate to the atmosphere.Any insulating films may be used as long as they assure a selectionratio with respect to silicon at subsequent etching. Silicon oxide,silicon nitride, silicon nitride oxide, or the like is used. In thisembodiment, the silicon nitride film of 1,000 Å by the plasma CVD methodis formed.

After forming the insulating film for the etching stopper 3020, thesubstrate 3001 is taken out into the atmosphere. With respect to thesubstrate 3001 taken out into the atmosphere, the insulating film forthe etching stopper 3020 is etched by a photolithographic technique toform the etching stopper 3020. This etching stopper 3020 determines achannel forming region of the TFT. As shown in the drawing, the etchingstopper is formed so that the width thereof is narrower than that of thegate electrode 3002. Next, by using the photolithographic technique, theactive layer and the gate insulating film 3004 are formed into desiredshapes. In this embodiment, although the active layer and the gateinsulating film 3004 are formed into the same shape, it is not requiredactually to form them into the same shape.

Next, impurities are doped into the source and drain regions by iondoping or ion implantation. In this embodiment, doping is carried out bythe ion doping. The ion doping is a method in which impurity doping iscarried out without mass separation. When doping of phosphorous iscarried out, PH3 is decomposed by plasma and is accelerated as it is, sothat doping is carried out into the source and drain regions of theactive layer. At this time, P ions and PH ions have no problem. However,since H ions easily pass through a film, an accelerating voltage must becontrolled so that the H ions do not reach the channel forming regionunder the etching stopper 3020.

By experiments, hydrogen ions by acceleration of 10 KeV intrude into thesilicon nitride film so that the peak of a doping amount exists at thedepth of about 1,800 Å. On the other hand, phosphorous ions by the sameacceleration of 10 KeV has the peak of a doping amount at the depth ofabout 300 Å. Accordingly, in this embodiment, doping is carried out byacceleration of 10 KeV at the dose amount of 5×10₁₄ to 5×10₁₅ atom/cm².

According to this doping, a source region 3030 and a drain region 3031are formed. Thereafter, a conductive film for a wiring layer is formed.As the conductive film, a metal film or a transparent conductive oxidefilm of ITO or the like is used. After forming the wiring layer, byusing the photolithographic technique, a source electrode 3040 and adrain electrode 3041 are formed so that the state shown in FIG. 3(A) isobtained. In the drawing, although the source and drain electrodes areshaped so that they extend to the upper portion of the etching stopper3020, this is not necessarily required in this embodiment. An interlayerinsulating film having contact holes may be formed so that the sourceelectrode 3040 and the source region 3030 are brought into contact witheach other through the contact hole, and the drain electrode 3041 andthe drain region 3031 may be brought into contact with each otherthrough the contact hole.

When the ion doping is carried out, the crystallinity of the dopedportion is lost so that the portion is converted into the amorphousstate. Accordingly, similar to the case where partial crystallization iscarried out, laser crystallization is again conducted. In this case,since the channel forming portion is covered with the etching stopper3020, the power of a laser and the number of shots can be easilyincreased to facilitate the crystallization.

The left side in FIG. 3(A) is the polysilicon TFT, and the right sidethereof is the amorphous silicon TFT. Although they are formed to beadjacent to each other in the drawing, the polysilicon TFT ispractically formed on a portion of the substrate where a driving circuitis formed, and the amorphous TFT is formed on a portion thereof where aswitching element for a pixel is formed.

FIG. 3(B) shows a modified embodiment almost equal to FIG. 3(A) in theprocess and structure. Reference numerals in FIG. 3(B) correspond tothose in FIG. 3(A). Only difference is that a gate insulating film isformed of two layers, that is, a first gate insulating film 3004 and asecond gate insulating film 3005 are provided.

Although it has been found by experiment that in the amorphous siliconTFT, a silicon nitride film is preferable as the gate insulating filmbeing contact with the channel forming region made of amorphous silicon.However, since the relative dielectric constant of the silicon nitridefilm is about as two times as large as that of the silicon oxide film,the thickness thereof becomes large. Thus, when only the portion beingcontact with the amorphous silicon is formed of the silicon nitridefilm, and other portion is formed of the silicon oxide film, the filmthickness of the entire gate insulating film can be made thin.

In this embodiment, the silicon oxide film of 1,000 Å is formed as thefirst gate insulating film 3004 by the plasma CVD method using organicsilane, and the silicon nitride film of 50 Å is formed as the secondgate insulating film 3005 by the plasma CVD method using a mixture gasof silane, ammonium and nitrogen.

Fourth Embodiment

Next, an example of a manufacturing apparatus for practicing the presentinvention will be described. FIG. 4(A) shows an apparatus for forming agate insulating film, forming an active layer, crystallizing the activelayer, and forming an insulating film for an etching stopper afterformation of a gate electrode without exposing the gate electrode to theatmosphere. Since all steps are carried out under a low pressure, theapparatus is formed into the structure of a multi-chamber type vacuumapparatus.

A common chamber 4000 including a robot for transferring a substrate andconnected to chambers for the respective steps is always in a lowpressure state. The common chamber 4000 is connected to the chambers forthe respective steps through the respective gates. A load chamber 4011and a load chamber gate 4010 are disposed, and a plurality of substratescan be set in the load chamber 4011, which has been opened to theatmosphere, under the state where the substrates are contained in acassette. After setting the substrates, the load chamber 4011 is madeinto a low pressure state. When a predetermined pressure is attained,the load chamber gate 4010 is opened and the robot in the common chamber4000 transfers the substrates one after another into the common chamber4000.

The transferred substrates are next transferred into a gate insulatingfilm forming chamber 4021. A gate insulating film forming chamber gate4020 is opened, and after the substrates are transferred into the gateinsulating film forming chamber, the gate 4020 is closed. In theformation of an insulating film, there may be used various methods suchas plasma CVD method, LPCVD method using lamp heating, sputteringmethod, and photo CVD method. In this embodiment, both the plasma CVDmethod and the photo CVD method can be carried out in the film formationchamber.

After the film is formed in the gate insulating film forming chamber4021, the substrates are transferred into an active layer formingchamber 4031. The active layer forming chamber 4031 is connected to thecommon chamber 4000 through an active layer forming chamber gate 4030.When the substrates go in and out the chamber, the active layer formingchamber gate 4030 opens and closes. In the film formation in the activelayer forming chamber 4031, there may be used various methods such asplasma CVD method, LPCVD method using lamp heating, sputtering method,and photo CVD method. In this embodiment, the active layer is formed bythe reactive sputtering. A single crystal silicon wafer is used as atarget for sputtering, and a sputtering gas is a mixture gas of argonand hydrogen. In the sputtering chamber, the substrates are treated oneafter another.

Next, the substrates are transferred into a crystallizing chamber 4041.The crystallizing chamber 4041 is connected to the common chamber 4000through a crystallizing chamber gate 4040. When the substrates go in andout the chamber, the crystallizing chamber gate 4040 opens and closes.The active layer of amorphous silicon of 100 ⊂ to 300 is partiallycrystallized using a laser or lamp.

Next, the substrates are transferred into an etching stopper chamber4051. The etching stopper chamber 4051 is connected to the commonchamber 4000 through an etching stopper chamber gate 4050.

When the substrates go in and out the chamber, the etching stopperchamber gate 4050 opens and closes. An insulating film for an etchingstopper is formed in the etching stopper chamber 4051. There are variousmethods such as plasma CVD method, LPCVD method using lamp heating,sputtering method, and photo CVD method. In this embodiment, the etchingstopper insulating film is formed by the plasma CVD.

Next, the substrates are transferred into an unload chamber 4061. Theunload chamber 4061 is connected to the common chamber 4000 through anunload chamber gate 4060. When the substrates go in and out the chamber,the unload chamber gate 4060 opens and closes. A cassette for containingthe substrates is disposed in the unload chamber 4061, and thesubstrates subjected to all the respective steps without being exposedto the atmosphere are put into the cassette. When the number of thesubstrates contained in the cassette reaches a predetermined number, theunload chamber 4061 is opened to the atmosphere and the substrates aretaken out.

FIG. 4(B) shows a method of partially crystallizing the substrate. Alaser oscillator 4200, a homogenizer 4300, and an optical system 4400are disposed for the substrate 4100. An excimer laser oscillator of KrF,XeCl or the like is used as the laser oscillator 4200. A laser beam fromthe oscillator is made to have a uniform energy surface by thehomogenizer 4300, and is shaped into a linear laser beam 4500 throughthe optical system 4400.

A portion of the substrate 4100 where crystallization is desired is onewhere a polysilicon TFT such as a driving circuit is required, and theportion is the vicinity of the end of the substrate 4100. Accordingly,if the substrate 4100 is rotated by 90° as shown by arrow 4600, onlynecessary portions can be crystallized.

More specifically, the portion where the polysilicon TFT is required isas shown in FIGS. 5(A) to 5(C). An amorphous silicon as an active layeris formed on the entire surface of a substrate 5000, and includes apixel region 5010, a source driver region 5020 mainly comprised of ashift register, buffer circuit, analog memory and the like, and a gatedriver region 5030 mainly comprised of a shift register.

The pixel region 5010 is a region where amorphous TFTs are formed. Thesource driver region 5020 and the gate driver region 5030 are regionswhere polysilicon TFTs are formed, and these are regions where thesubstrate 5000 is partially crystallized. The irradiation of an excimerlaser beam shaped into a linear laser beam is first carried out to thegate driver region 5030 so that this region 5030 is converted into apolysilicon gate driver region 5031. Next, the substrate is rotated by90° and the irradiation of the excimer laser beam is carried out to thesource driver region 5020 to form a polysilicon source driver region5021 so that the partial crystallization is completed.

Various kinds of circuit such as a shift register, buffer circuit, andanalog memory are formed in the source driver region. When lasercrystallization is carried out, if the entire region is crystallized atonce, there is no problem. However, when crystallization is carried outby repeating division irradiation in the source driver region, it hasbeen found by experiment that irregularity of characteristics oftransistors is small when division is carried out for the respectivecircuits, not for places on the substrate. This is because theoscillator of the excimer laser was not stable at the time when thepresent invention was made. Since it is preferable that thecharacteristics of crystallized silicon is uniform in the same circuit,it is necessary to design the circuit arrangement so that irradiationcan be carried out for the respective circuits. It is supposed that ifthe oscillator of the excimer laser becomes stable, such a problembecomes less serious.

FIG. 6 shows an apparatus for carrying out the partial crystallizationof the invention under a low pressure. FIG. 6(A) shows an example ofusing lamp heating. A substrate 6000 is placed on a substrate holder6010, and a mask 6100 is disposed over the substrate 6000. A lampchamber 6200 is dome-shaped, and a halogen lamp is set.

An inner wall of the chamber is covered with a reflective plate so thatthe substrate is irradiated with the light from the lamp. The light isreflected as shown by arrows 6201 and then applied or directly appliedto the substrate. A gate 6310 and a vacuum pump 6320 are connected to acrystallizing chamber 6300, and a nozzle 6400 and a nozzle cover 6410for introducing a gas such as hydrogen are disposed in the crystallizingchamber 6300.

The halogen lamp has an output of 1,000 to 3,000 W so that it caninstantaneously crystallize the amorphous silicon on the substrate 6000.A portion where crystallization is not desired is covered with a mask6100, and the portion is not irradiated with the lamp light so that itis not crystallized. In view of curvature, it is preferable to use fineceramics as a material of the mask 6100. In this embodiment, an aluminumplate is used.

FIG. 6(B) shows an example using a laser. A substrate 6500 is placed ona substrate holder 6510 and is placed in a crystallizing chamber 6800. Agate 6810 and a vacuum pump 6820 are connected to the crystallizingchamber 6800. Further, a window 6600 for making a laser beam passthrough is attached to the chamber 6800. The window 6600 is made ofquartz or synthetic quartz, and is designed to allow the wavelength ofnot larger than 400 nm to pass through. A laser oscillator 6700, ahomogenizer 6710, and an optical system 6720 are disposed outside thecrystallizing chamber 6800 so that the laser beam can be shaped into alinear beam 6730.

When the crystallizing chamber as shown in FIGS. 6(A) or 6(B) is used asone of multi-chamber processing chambers as shown in FIG. 4(A),formation of the insulating gate film, formation of the active layer,partial crystallization of the active layer, and formation of theetching stopper film can be continuously carried out. Also, if animpurity semiconductor film forming chamber and a conductive filmforming chamber are provided instead of the film forming chamber of theetching stopper film, it is possible to carry out a step as shown inFIG. 2 in which the channel forming region is slightly etched.

According to the present invention, the amorphous silicon TFT andpolysilicon TFT of bottom gate type and reverse stagger type can beformed on the same substrate. Accordingly, a driver circuit having largefrequency characteristics and also capable of being formed into acomplementary type can be made by forming a driver circuit portion ofpolysilicon, and at the same time, a switching element having a smallOFF-state leak current can be fabricated by the amorphous silicon TFTfor a pixel portion.

Further, when process chambers of the respective steps are disposed inthe multi-chamber, almost all portions of a transistor can be formedwithout being exposed to the atmosphere. Thus, the characteristics ofthe transistor can be improved. When crystallization is carried outusing a laser, partial crystallization can be easily carried out byrotating the substrate by 90°. When crystallization is carried out byusing a lamp, partial crystallization can be easily carried out by usinga mask. Accordingly, the present invention is of high utility value intechnology.

What is claimed is:
 1. A semiconductor device comprising: at least onefirst thin film transistor formed over a substrate; a driving circuitcomprising at least one second thin film transistor formed over saidsubstrate, said second thin film transistor comprising: a gate electrodecomprising chromium formed over the substrate where in said gateelectrode has tapered side edges; a gate insulating film formed on saidgate electrode; a semiconductor film comprising crystalline siliconformed on said gate insulating film where in said semiconductor filmincludes a channel forming region over said gate electrode, and a pairof impurity doped regions with the channel forming region interposedtherebetween, wherein said semiconductor film has a thickness in a rangefrom 100 to 300 Å.
 2. A semiconductor device according to claim 1wherein said substrate is selected from the group consisting of sodaglass, borosilicate glass and quartz.
 3. A semiconductor deviceaccording to claim 1 wherein said substrate is an insulating substratecoated with a film comprising a material selected from the groupconsisting of PSG, silicon oxide, and silicon nitride.
 4. Asemiconductor device according to claim 1 wherein said gate insulatingfilm has a multi-layer structure including at least a first layercomprising silicon oxide and a second layer comprising silicon nitride,and said second layer is in contact with said semiconductor film.
 5. Asemiconductor device according to claim 1 wherein said semiconductorfilm is formed by a method comprising steps of depositing an amorphoussemiconductor film on said gate insulating film and crystallizing saidamorphous semiconductor film by irradiation of pulsed excimer laserbeam.
 6. A semiconductor device according to claim 1 wherein aninsulating film is formed on said channel forming region as an etchingstopper.
 7. A semiconductor device according to claim 1 wherein saidgate insulating film has a thickness within a range from 1000 to 3000 Å.8. A semiconductor device according to claim 1 wherein saidsemiconductor film is formed without exposing a surface of said gateinsulating film after the formation of said gate insulating film.
 9. Asemiconductor device according to claim 1 wherein said semiconductordevice is an active matrix liquid crystal display device.
 10. Asemiconductor device comprising: at least one first thin film transistorformed in a pixel portion over a substrate; a driving circuit includingat least one circuit selected from the group consisting of a shiftregister, a buffer circuit and an analog memory, said circuit comprisingat least one second thin film transistor formed over said substrate,said second thin film transistor comprising: a gate electrode comprisingchromium formed over the substrate, wherein said gate electrode hastapered side edges; a gate insulating film formed on said gateelectrode; a semiconductor film comprising crystalline silicon formed onsaid gate insulating film wherein said semiconductor film includes achannel forming region over said gate electrode, and a pair of impuritydoped regions with the channel forming region interposed therebetween,wherein said substrate has a level upper surface.
 11. A semiconductordevice according to claim 10 wherein said substrate is selected from thegroup consisting of soda glass, borosilicate glass and quartz.
 12. Asemiconductor device according to claim 10 wherein said substrate is aninsulating substrate coated with a film comprising a material selectedfrom the group consisting of PSG, silicon oxide, and silicon nitride.13. A semiconductor device according to claim 10 wherein said gateinsulating film has a multi-layer structure including at least a firstlayer comprising silicon oxide and a second layer comprising siliconnitride, and said second layer contacts said semiconductor film.
 14. Asemiconductor device according to claim 10 wherein said semiconductorfilm is formed by a method comprising steps of depositing an amorphoussemiconductor film on said gate insulating film and crystallizing saidamorphous semiconductor film by irradiation of pulsed excimer laserbeam.
 15. A semiconductor device according to claim 10 wherein aninsulating film is formed on said channel forming region as an etchingstopper.
 16. A semiconductor device according to claim 10 wherein saidgate insulating film has a thickness within a range from 1000 to 3000 Å.17. A semiconductor device according to claim 10 wherein saidsemiconductor film is formed without exposing a surface of said gateinsulating film after the formation of said gate insulating film.
 18. Asemiconductor device according to claim 10 wherein said semiconductordevice is an active matrix liquid crystal display device.
 19. Asemiconductor device having at least one thin film transistor, said thinfilm transistor comprising: a gate electrode formed over a substratewherein said gate electrode has tapered side edges; a gate insulatingfilm formed on said gate electrode; a semiconductor film comprisingcrystalline silicon formed on said gate insulating film wherein saidsemiconductor film includes a channel forming region over said gateelectrode, a pair of impurity regions with the channel forming regioninterposed therebetween and at least one lightly doped region interposedbetween said channel forming region and one of said pair of impurityregions wherein said lightly doped region is doped with an impurity at alower concentration than said pair of impurity regions, wherein saidgate insulating film has a multi-layer structure including at least afirst layer comprising silicon oxide and a second layer comprisingsilicon nitride, and said second layer contacts said se miconductorfilm.
 20. A semiconductor device according to claim 19 wherein saidsubstrate is selected from the group consisting of soda glass,borosilicate glass and quartz.
 21. A semiconductor device according toclaim 19 wherein said substrate is an insulating substrate coated with afilm comprising a material selected from the group consisting of PSG,silicon oxide, and silicon nitride.
 22. A semiconductor device accordingto claim 19 wherein said semiconductor film is formed by a methodcomprising steps of depositing an amorphous semiconductor film on saidgate insulating film and crystallizing said amorphous semiconductor filmby irradiation of pulsed excimer laser beam.
 23. A semiconductor deviceaccording to claim 19 wherein an insulating film is formed on saidchannel forming region as an etching stopper.
 24. A semiconductor deviceaccording to claim 19 wherein said gate insulating film has a thicknesswithin a range from 1000 to 3000 Å.
 25. A semiconductor device accordingto claim 19 wherein said semiconductor device is an active matrix liquidcrystal display device.
 26. A semiconductor device comprising: at leastone first thin film transistor formed in a pixel portion over asubstrate; a driving circuit comprising at least one second thin filmtransistor formed over said substrate, said second thin film transistorcomprising: a gate electrode formed over the substrate wherein said gateelectrode has tapered side edges; a gate insulating film formed on saidgate electrode; a semiconductor film comprising crystalline siliconformed on said gate insulating film wherein said semiconductor filmincludes a channel forming region over said gate electrode, a pair ofimpurity regions with the channel forming region interposed therebetweenand at least one lightly doped region interposed between said channelforming region and one of said pair of impurity regions wherein saidlightly doped region is doped with an impurity at a lower concentrationthan said pair of impurity regions, wherein said semiconductor film hasa thickness of 100 to 300 Å.
 27. A semiconductor device according toclaim 26 wherein said substrate is selected from the group consisting ofsoda glass, borosilicate glass and quartz.
 28. A semiconductor deviceaccording to claim 26 wherein said substrate is an insulating substratecoated with a film comprising a material selected from the groupconsisting of PSG, silicon oxide, and silicon nitride.
 29. Asemiconductor device according to claim 26 wherein said gate insulatingfilm has a multi-layer structure including at least a first layercomprising silicon oxide and a second layer comprising silicon nitride,wherein said second layer is in contact with said semiconductor film.30. A semiconductor device according to claim 26 wherein saidsemiconductor film is formed by a method comprising steps of depositingan amorphous semiconductor film on said gate insulating film andcrystallizing said amorphous semiconductor film by irradiation of pulsedexcimer laser beam.
 31. A semiconductor device according to claim 26wherein an insulating film is formed on said channel forming region asan etching stopper.
 32. A semiconductor device according to claim 26wherein said gate insulating film has a thickness within a range from1000 to 3000 Å.
 33. A semiconductor device according to claim 26 whereinsaid semiconductor device is an active matrix liquid crystal displaydevice.
 34. A semiconductor device comprising: at least one first thinfilm transistor formed in a pixel portion over a substrate; a drivingcircuit including at least one circuit selected from the groupconsisting of a shift register, a buffer circuit and an analog memory,said circuit comprising a plurality of second thin film transistorsformed over said substrate, at least one of said second thin filmtransistors comprising: a gate electrode formed over the substratewherein said gate electrode has tapered side edges; a gate insulatingfilm formed on said gate electrode; a semiconductor film comprisingcrystalline silicon formed on said gate insulating film wherein saidsemiconductor film includes a channel forming region over said gateelectrode, a pair of impurity regions with the channel forming regioninterposed therebetween and at least one lightly doped region interposedbetween said channel forming region and one of said pair of impurityregions wherein said lightly doped region is doped with an impurity at alower concentration than said pair of impurity regions, wherein saidsemiconductor film has a thickness of 100 to 300 Å.
 35. A semiconductordevice according to claim 34 wherein said substrate is selected from thegroup consisting of soda glass, borosilicate glass and quartz.
 36. Asemiconductor device according to claim 34 wherein said substrate is aninsulating substrate coated with a film comprising a material selectedfrom the group consisting of PSG, silicon oxide, and silicon nitride.37. A semiconductor device according to claim 34 wherein said gateinsulating film has a multi-layer structure including at least a firstlayer comprising silicon oxide and a second layer comprising siliconnitride, wherein said second layer is in contact with said semiconductorfilm.
 38. A semiconductor device according to claim 34 wherein saidsemiconductor film is formed by a method comprising steps of depositingan amorphous semiconductor film on said gate insulating film andcrystallizing said amorphous semiconductor film by irradiation of pulsedexcimer laser beam.
 39. A semiconductor device according to claim 34wherein an insulating film is formed on said channel forming region asan etching stopper.
 40. A semiconductor device according to claim 34wherein said gate insulating film has a thickness within a range from1000 to 3000 Å.
 41. A semiconductor device according to claim 34 whereinsaid semiconductor device is an active matrix liquid crystal displaydevice.
 42. A semiconductor device comprising: at least one first thinfilm transistor formed in a pixel portion over a substrate; a drivingcircuit including at least one circuit selected from the groupconsisting of a shift register, a buffer circuit and an analog memory,said circuit comprising at least one second thin film transistors formedover said substrate, each of said first and second thin film transistorscomprising: a gate electrode formed over the substrate; a gateinsulating film formed over said gate electrode; a semiconductor filmformed on said gate insulating film, said semiconductor film including achannel forming region over said gate electrode, a pair of impurityregions with the channel forming region interposed therebetween, whereinsaid semiconductor film has a thickness of 100 to 300 Å, wherein thechannel forming region of said first thin film transistor comprisesamorphous silicon while the channel forming region of said second thinfilm transistor comprises crystalline silicon.
 43. A semiconductordevice according to claim 42 wherein said substrate is selected from thegroup consisting of soda glass, borosilicate glass and quartz.
 44. Asemiconductor device according to claim 42 wherein said substrate is aninsulating substrate coated with a film comprising a material selectedfrom the group consisting of PSG, silicon oxide, and silicon nitride.45. A semiconductor device according to claim 42 wherein said gateinsulating film has a multi-layer structure including at least a firstlayer comprising silicon oxide and a second layer comprising siliconnitride, wherein said second layer is in contact with said semiconductorfilm.
 46. A semiconductor device according to claim 42 wherein said gateinsulating film has a thickness within a range form 1000 to 3000 Å. 47.A semiconductor device according to claim 42 wherein said semiconductorfilm of the second thin film transistor further includes at least onelightly doped region interposed between said channel forming region andone of said pair of impurity regions wherein said lightly doped regionis doped with an impurity at a lower concentration than said pair ofimpurity regions.
 48. A semiconductor device according to claim 42wherein said semiconductor device is an active matrix liquid crystaldisplay device.
 49. A semiconductor device comprising: at least onefirst thin film transistor formed in a pixel portion over a substrate; adriving circuit including at least one circuit selected from the groupconsisting of a shift register, a buffer circuit and an analog memory,said circuit comprising at least one second thin film transistors formedover said substrate, each of said first and second thin film transistorscomprising: a gate electrode formed over the substrate; a gateinsulating film formed over said gate electrode; a semiconductor filmformed on said gate insulating film, said semiconductor film including achannel forming region over said gate electrode, a pair of impurityregions with the channel forming region interposed therebetween, whereinsaid semiconductor film has a thickness of 100 to 300 Å, wherein thesemiconductor film of said second thin film transistor has a higherdegree of crystallinity than the semiconductor film of said first thinfilm transistor that is not crystallized.
 50. A semiconductor deviceaccording to claim 49 wherein said substrate is an insulating substratecoated with a film comprising a material selected from the groupconsisting of PSG, silicon oxide, and silicon nitride.
 51. Asemiconductor device according to claim 49 wherein said gate insulatingfilm has a multi-layer structure including at least a first layercomprising silicon oxide and a second layer comprising silicon nitride,wherein said second layer is in contact with said semiconductor film.52. A semiconductor device according to claim 49 wherein said gateinsulating film has a thickness within a range from 1000 to 3000 Å. 53.A semiconductor device according to claim 49 wherein said semiconductorfilm of the second thin film transistor further includes at lest onelightly doped region interposed between said channel forming region andone of said pair of impurity regions wherein said lightly doped regionis doped with an impurity at a lower concentration than said pair ofimpurity regions.
 54. A semiconductor device according to claim 49wherein said semiconductor device is an active matrix liquid crystaldisplay device.
 55. A semiconductor device comprising: at least onefirst thin film transistor formed in a pixel portion over a substrate; adriving circuit comprising at least one second thin film transistorformed over said substrate, said second thin film transistor comprising:a gate electrode comprising chromium formed over said substrate whereinsaid gate electrode has tapered side edges; a gate-insulating filmformed on said gate electrode; and a semiconductor film comprisingcrystalline silicon formed on said gate insulating film wherein saidsemiconductor film includes a channel forming region over said gateelectrode, a pair of impurity regions with said channel forming regioninterposed therebetween and at least one lightly doped drain regioninterposed between said channel forming region and one of said pair ofimpurity regions wherein said lightly doped region is doped with animpurity at a lower concentration than said pair of impurity region. 56.A semiconductor device according to claim 55 wherein said substrate isselected from the group consisting of soda glass, borosilicate glass andquartz.
 57. A semiconductor device according to claim 55 wherein saidsubstrate is an insulating substrate coated with a film comprising amaterial selected from the group consisting of PSG, silicon oxide, andsilicon nitride.
 58. A semiconductor device according to claim 55wherein said gate insulating film has a multi-layer structure includingat least a first layer comprising silicon oxide and a second layercomprising silicon nitride, wherein said second layer is in contact withsaid semiconductor film.
 59. A semiconductor device according to claim55 wherein said semiconductor film is formed by a method comprisingsteps of depositing an amorphous semiconductor film on said gateinsulating film and crystallizing said amorphous semiconductor film byirradiation of pulsed excimer laser beam.
 60. A semiconductor deviceaccording to claim 55 wherein an insulating film is formed on saidchannel forming region as an etching stopper.
 61. A semiconductor deviceaccording to claim 55 wherein said gate insulating film has a thicknesswithin a range from 1000 to 3000 Å.
 62. A semiconductor device accordingto claim 55 wherein said semiconductor device is an active matrix liquidcrystal display device.
 63. A semiconductor device comprising: at leastone first thin film transistor formed in a pixel portion over asubstrate; a driving circuit including at least one circuit selectedfrom the group consisting of a shift resister, a buffer circuit and ananalog circuit memory, said circuit comprising a plurality of secondthin film transistors formed over said substrate, at least one of saidsecond thin film transistors comprising: a gate electrode comprisingchromium formed over said substrate wherein said gate electrode hastapered side edges; a gate insulating film formed on said gateelectrode; and a semiconductor film comprising crystalline siliconformed on said gate insulating film wherein said semiconductor filmincludes a channel forming region over said gate electrode, a pair ofimpurity regions with said channel forming region interposedtherebetween and at least one lightly doped drain region interposedbetween said channel forming region and one of said pair of impurityregions wherein said lightly doped region is doped with an impurity at alower concentration than said pair of impurity region.
 64. Asemiconductor device according to claim 63 wherein said substrate isselected from the group consisting of soda glass, borosilicate glass andquartz.
 65. A semiconductor device according to claim 63 wherein saidsubstrate is an insulating substrate coated with a film comprising amaterial selected from the group consisting of PSG, silicon oxide, andsilicon nitride.
 66. A semiconductor device according to claim 63wherein said gate insulating film has a multi-layer structure includingat least a first layer comprising silicon oxide and a second layercomprising silicon nitride, wherein said second layer is in contact withsaid semiconductor film.
 67. A semiconductor device according to claim63 wherein said semiconductor film is formed by a method comprisingsteps of depositing an amorphous semiconductor film on said gateinsulating film and crystallizing said amorphous semiconductor film byirradiation of pulsed excimer laser beam.
 68. A semiconductor deviceaccording to claim 63 wherein an insulating film is formed on saidchannel forming region as an etching stopper.
 69. A semiconductor deviceaccording to claim 63 wherein said gate insulating film has a thicknesswithin a range from 1000 to 3000 Å.
 70. A semiconductor device accordingto claim 63 wherein said semiconductor device is an active matrix liquidcrystal display device.